1. Field of the Invention
This invention relates to a green emitting fluorescent composition and a cathode ray tube for color television using the same as a green emitting phosphor component, and more particularly to a green emitting fluorescent composition of a mixture of a copper and aluminium activated zinc sulfide phosphor and a gold and aluminium activated zinc sulfide phosphor used as a green emitting phosphor component for the fluorescent screen of a color television cathode ray tube and also to a color television cathode ray tube using the same.
2. Description of the Prior Art
The fluorescent screen of a color television cathode ray tube is comprised of a number of dots or stripes of green, blue and red emitting phosphor components and presents various colors of light when excited by an electron beam. In the conventional color television cathode ray tubes, there have been used a copper and aluminium activated zinc cadmium sulfide phosphor [(Zn,Cd)S:Cu,Al] as a green emitting phosphor component, a silver activated zinc sulfide phosphor (ZnS:Ag) or a silver and aluminium activated zinc sulfide phosphor (ZnS:Ag,Al) as a blue emitting phosphor component, and a europium activated yttrium oxysulfide phosphor (Y.sub.2 O.sub.2 S:Eu) or a europium activated yttrium oxide phosphor (Y.sub.2 O.sub.3 :Eu) or a europium activated yttrium vanadate phosphor (YVO.sub.4 :Eu) as a red emitting phosphor component.
As shown in the above description, cadmium is an essential element for the fluorescent screen of a color television cathode ray tube. However, cadmium is a highly noxious metal, and accordingly requires a great care in handling the same in the process of coating the fluorescent screen in the cathode ray tube. Further, the cathode ray tube using cadmium in its fluorescent screen requires a great care in handling the same when it is broken or destroyed. In addition, since the allowable concentration of cadmium in waste materials is severly limited by the anti-polution law, a large scaled equipment is needed to treat the waste materials. Thus, it is economically very disadvantageous to use cadmium in the fluorescent screen.
Under the above described circumstances, it has been known to use a copper and aluminium activated zinc sulfide phosphor (ZnS:Cu,Al) mainly of cubic structure instead of the copper and aluminium activated zinc cadmium sulfide phosphor (Zn,Cd)S:Cu,Al. However, the copper and aluminium activated zinc sulfide phosphor (ZnS:Cu,Al) mainly of cubic structure (hereinafter referred to simply as "cubic ZnS:Cu,Al phosphor") lowers the white light luminance. The luminance of the color cathode ray tube, namely the brightness, is generally evaluated by the white light luminance represented by the luminance of a white point (x=0.281, y=0.311) in the JEDEC (Joint Election Device Engineering Councils) standard. In the conventional copper and aluminium activated zinc cadmium sulfide used as a green emitting phosphor component in the conventional color television cathode ray tube, the spectrum of the emitted light can easily be made to be in the long wavelength side of green by increasing the amount of cadmium which constitutes the host material of the phosphor. Actually, in the conventional color television cathode ray tube, the (Zn,Cd)S:Cu,Al phosphor which emits green light biased to yellow is used, and accordingly it has been possible to increase the share of the exciting current for the green emitting phosphor component which has a high emission efficiency in comparison with the red emitting phosphor component. Therefore, a very high white luminance is obtainable. On the other hand, in the cubic ZnS:Cu,Al phosphor which does not contain cadmium, the green light emitted is biased to the short wavelength side and it is very difficult to shift the color of the emitted light to yellow side beyond the color point of (x=0.300, y=0.630) in the CIE standard chromaticity diagram. Therefore, when this phosphor is used as the green emitting phosphor component, the share of the exciting current for the red emitting phosphor component which has a low emission efficiency in comparison with the green emitting phosphor component is inevitably increased in comparison with the conventional (Zn,Cd)S:Cu,Al phosphor. Consequently, the white luminance is lowered about 10%.
In view of the above described state of the art, it is desired to provide a green emitting phosphor for a color television cathode ray tube which does not contain noxious cadmium like the cubic ZnS:Cu,Al phosphor, emits light of the wavelength biased to the long wavelength side more than the cubic ZnS:Cu,Al phosphor, and results in increase in the white luminance in comparison with the case wherein the cubic ZnS:Cu,Al phosphor is used.